U.S. patent application number 12/755768 was filed with the patent office on 2010-10-14 for varactor element and electronic device.
This patent application is currently assigned to SONY CORPORATION. Invention is credited to Kazutaka Habu, Masayoshi Kanno, Noritaka Sato.
Application Number | 20100259864 12/755768 |
Document ID | / |
Family ID | 42934193 |
Filed Date | 2010-10-14 |
United States Patent
Application |
20100259864 |
Kind Code |
A1 |
Kanno; Masayoshi ; et
al. |
October 14, 2010 |
VARACTOR ELEMENT AND ELECTRONIC DEVICE
Abstract
A varactor element, includes: a dielectric layer; a pair of
signal electrodes, each disposed on one face of the dielectric
layer and facing each other; and a pair of control electrodes, each
disposed on another face of the dielectric layer and facing each
other parallel to a direction intersecting a direction of the pair
of signal electrodes facing each other.
Inventors: |
Kanno; Masayoshi; (Kanagawa,
JP) ; Habu; Kazutaka; (Tokyo, JP) ; Sato;
Noritaka; (Kanagawa, JP) |
Correspondence
Address: |
K&L Gates LLP
P. O. BOX 1135
CHICAGO
IL
60690
US
|
Assignee: |
SONY CORPORATION
Tokyo
JP
|
Family ID: |
42934193 |
Appl. No.: |
12/755768 |
Filed: |
April 7, 2010 |
Current U.S.
Class: |
361/277 |
Current CPC
Class: |
H01L 28/60 20130101;
H01L 27/0808 20130101; H01L 29/93 20130101 |
Class at
Publication: |
361/277 |
International
Class: |
H01G 7/00 20060101
H01G007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 13, 2009 |
JP |
P2009-097276 |
Claims
1. A varactor element, comprising: a dielectric layer; a pair of
signal electrodes, each disposed on one face of the dielectric
layer and facing each other; and a pair of control electrodes, each
disposed on another face of the dielectric layer and facing each
other parallel to a direction intersecting a direction of the pair
of signal electrodes facing each other.
2. The varactor element according to claim 1, wherein a distance
between the pair of signal electrodes is different from a distance
between the pair of control electrodes.
3. The varactor element according to claim 2, wherein the distance
between the pair of signal electrodes is larger than the distance
between the pair of control electrodes.
4. The varactor element according to claim 1, wherein the
dielectric layer is formed of a dielectric material having a
permittivity varied by an application of a direct current
voltage.
5. A varactor element, comprising: a pair of signal electrodes
disposed facing each other parallel to a predetermined direction; a
pair of control electrodes disposed facing each other parallel to a
direction intersecting the predetermined direction; a first
dielectric layer disposed between the pair of signal electrodes and
disposed between the pair of control electrodes; and a second
dielectric layer having a permittivity lower than that of the first
dielectric layer and disposed between the signal electrodes and the
control electrodes.
6. The varactor element according to claim 5, wherein the pair of
signal electrodes, the pair of control electrodes, and the second
dielectric layer are disposed parallel to an in-plane direction of
the first dielectric layer.
7. The varactor element according to claim 5, wherein one of the
pair of signal electrodes and one of the pair of control electrodes
are disposed parallel to an in-plane direction of the first
dielectric layer, another of the signal electrodes and another of
the control electrodes are disposed parallel to a thickness of the
first dielectric layer, and the second dielectric layer is disposed
between the signal electrodes and the control electrodes.
8. An electronic device, comprising: a varactor element having a
dielectric layer, a pair of signal electrodes, each disposed on one
face of the dielectric layer and facing each other, and a pair of
control electrodes, each disposed on another face of the dielectric
layer and facing each other parallel to a direction intersecting a
direction of the pair of signal electrodes facing each other; and a
control voltage supply unit supplying a control voltage to the pair
of control electrodes.
9. An electronic device, comprising: a varactor element having a
pair of signal electrodes disposed facing each other parallel to a
predetermined direction, a pair of control electrodes disposed
facing each other parallel to a direction intersecting the
predetermined direction, a first dielectric layer disposed between
the pair of signal electrodes and disposed between the pair of
control electrodes, and a second dielectric layer having a
permittivity lower than that of the first dielectric layer and
disposed between the signal electrodes and the control electrodes;
and a control voltage supply unit supplying a control voltage to
the pair of control electrodes.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] The present application claims priority to Japanese Priority
Patent Application JP 2009-097276 filed in the Japan Patent Office
on Apr. 13, 2009, the entire content of which is hereby
incorporated by reference.
BACKGROUND
[0002] The present application relates to a varactor element and an
electronic device provided with the same, and more specifically, to
a varactor element that changes the capacitance by applying a
control field and an electronic device provided with the same.
[0003] In the past, varactor elements are utilized that apply a
bias signal from outside and change the capacitance to control
frequencies, time, and the like. As such varactor elements,
varactor diodes (variable capacitance diodes) and MEMS (micro
electro mechanical systems) are commercialized, for example. Such a
varactor element used for these applications usually has two
terminals and does not have a terminal exclusively for application
of a controlling bias signal that controls the capacitance.
Therefore, in an actual circuit, such two-terminal varactor
elements are arranged to function as a four-terminal element.
[0004] FIGS. 27A and 27B illustrate an example of a circuit
configuration when a two-terminal varactor element is arranged to
function as a four-terminal element. In an example illustrated in
FIG. 27A, a varactor element 160 (varactor capacitor) has one of
the terminals connected to one of the input/output terminals of the
alternating current signal via a bias removal capacitor 161 and
also connected to an input terminal of the control voltage via a
current limiting resistor 162. The varactor element 160 has the
other terminal connected to the other input/output terminal of the
alternating current signal and also connected to an output terminal
of the control voltage.
[0005] In the circuit configuration illustrated in FIG. 27A, the
signal current (alternating current signal) flows in the bias
removal capacitor 161 and the varactor capacitor 160 and the
control current (direct bias current) flows only in the varactor
capacitor 160 via the current limiting resistor 162. At this point,
by changing the control voltage, the capacitance of the varactor
capacitor 160 is changed, and as a result, the signal current is
also changed.
[0006] In an example illustrated in FIG. 27B, the varactor element
160 has one of the terminals, similar to FIG. 27A, connected to one
of the input/output terminals of the alternating current signal via
the bias removal capacitor 161 and also connected to an input
terminal of the control voltage via the current limiting resistor
162. In the example illustrated in FIG. 27B, the varactor element
160 has the other terminal connected to the other input/output
terminal of the alternating current signal via a bias removal
capacitor 163 and also connected to an output terminal of the
control voltage via a current limiting resistor 164. That is, in
the example illustrated in FIG. 27B, the peripheral circuitry
configuration connected to the formerly mentioned one terminal of
the varactor element 160 is also applied to the other terminal.
[0007] In the circuit configuration illustrated in FIG. 27B,
similar to the example illustrated in FIG. 27A, the signal current
flows in the two bias removal capacitors 161 and 163 and also the
varactor capacitor 160, and the control current flows only in the
varactor capacitor 160. Therefore, in the example illustrated in
FIG. 27B as well, the capacitance of the varactor capacitor 160 is
also changed by changing the control voltage, and as a result, the
signal current is also changed.
[0008] However in the circuit configuration illustrated in FIGS.
27A and 27B, while the control voltage source and the signal
voltage source are configured individually, the terminal of the
varactor element 160 to which they are finally connected is in
common. In this case, although the two terminals are arranged to
function as four terminals in the circuit, the direct bias current
(control current) flowing from the control voltage source
interferes with the signal current (refer to arrows in broken lines
in FIG. 27B, for example).
[0009] Therefore, the circuit configuration illustrated in FIGS.
27A and 27B uses the current limiting resistor 162 and/or 164 for
protection and/or separation of the control circuit, and uses the
bias removal capacitor 161 and/or 163 for protection and/or
separation of the signal circuit. The resistance value R is in
particular established to be large for secure protection and/or
separation of the control circuit. However, in this case, a time
constant (=RC) determined by the resistance value R of the current
limiting resistor 162 and/or 164 and the capacitance C of the
varactor element 160 becomes large and the responsiveness of
capacitance control is decreased.
[0010] Although the varactor elements illustrated in FIGS. 27A and
27B are provided with the bias removal capacitor(s) as they are
substantially two-terminal elements, the present inventors already
proposed a varactor element of a configuration without using such a
bias removal capacitor (refer to Japanese Unexamined Patent
Application Publication No. 2007-287996). In Japanese Unexamined
Patent Application Publication No. 2007-287996, an element is
proposed that uses a ferroelectric material as a varactor element.
FIGS. 28A and 28B illustrate the electrode structure of a varactor
element 200 proposed in Japanese Unexamined Patent Application
Publication No. 2007-287996. FIG. 28A is a schematic perspective
view of the varactor element 200, and FIG. 28B is a cross-sectional
configuration diagram of a dielectric member 204 configuring the
varactor element 200.
[0011] In the varactor element 200 according to Japanese Unexamined
Patent Application Publication No. 2007-287996, respective
terminals are provided on four faces of the dielectric member 204
in a rectangular parallelepiped shape. Out of the four terminals,
two of the facing terminals are signal terminals 203a and 203b
connected to a signal power source 203 and the other two facing
terminals are control terminals 202a and 202b connected to a
control power source 202.
[0012] Inside the varactor element 200, as illustrated in FIG. 28B,
has a structure in which a plurality of control electrodes 202c
through 202g and a plurality of signal electrodes 203c through 203f
are alternately laminated via a ferroelectric layer 205. In the
example of FIG. 28B, the control electrode 202g of the lowermost
layer, the fifth control electrode 202e from the bottom, and the
control electrodes 202c of the uppermost layer in the drawing are
connected to one of the control terminals, 202a. The third control
electrode 202f from the bottom and the seventh control electrode
202d from the bottom are connected to the other control terminal
202b. The fourth signal electrode 203e from the bottom and the
eighth signal electrode 203c from the bottom are connected to one
of the signal terminals, 203a. The second signal electrode 203f
from the bottom and the sixth signal electrode 203d from the bottom
are connected to the other signal terminal 203b.
[0013] The varactor element 200 according to Japanese Unexamined
Patent Application Publication No. 2007-287996 has a configuration
of separately providing the control terminals and the signal
terminals to separately apply the control voltage or the signal
voltage to the respective terminals. The varactor element 200
according to Japanese Unexamined Patent Application Publication No.
2007-287996 also has a configuration in which the plurality of
signal electrodes and control electrodes are laminated inside the
dielectric member 204. Therefore, the varactor element 200 of
Japanese Unexamined Patent Application Publication No. 2007-287996
has an advantage of allowing the capacitance to be increased at low
costs. Further, the varactor element 200 of a structure such as in
Japanese Unexamined Patent Application Publication No. 2007-287996
is manufactured easily at low costs. The varactor element 200
according to Japanese Unexamined Patent Application Publication No.
2007-287996 also has an advantage of working without a bias removal
capacitor.
SUMMARY
[0014] As described above, since the signal electrodes and the
control electrodes are alternately laminated via the ferroelectric
layer 205 in the varactor element 200 of Japanese Unexamined Patent
Application Publication No. 2007-287996, adjacent electrodes are
capacitively coupled to each other as illustrated by capacitors C1
through C8 in FIG. 28B. As a result, a signal capacitor formed
between the pair of signal terminals 203a and 203b and a
controlling capacitor formed between the pair of control terminals
202a and 202b turn out to be directly connected to each other in
the dielectric member 204. At this time, the direction of a signal
field generated between the pair of signal terminals 203a and 203b
becomes same as the direction of a control field generated between
the pair of control terminals 202a and 202b. FIG. 29 illustrates
such a behavioral situation in the dielectric member 204 more
schematically.
[0015] Although the directions of the pair of signal terminals 203a
and 203b facing each other are perpendicular to the directions of
the pair of control terminals 202a and 202b facing each other in
the varactor element 200 of Japanese Unexamined Patent Application
Publication No. 2007-287996, the signal electrodes and the control
electrodes are alternately laminated via the ferroelectric layer
205 inside the dielectric member 204. Therefore, the varactor
element 200 has a configuration substantially equivalent to a
configuration in which the pair of signal terminals 203a and 203b
and the pair of control terminals 202a and 202b are disposed in an
identical direction as illustrated in FIG. 29. The varactor element
200 also has a configuration in which the pair of control terminals
202a and 202b is connected with a variable capacitor and the signal
terminals and the control terminals are connected with fixed
(constant capacitance) capacitors. In this case, the direction of a
signal field generated between the pair of signal terminals 203a
and 203b becomes same as the direction of a control field generated
between the pair of control terminals 202a and 202b (direction of
an arrow in a broken line in FIG. 29).
[0016] In the varactor element 200 of the configuration described
above in which the direction of the signal field conforms to the
direction of the control field, in order to change the capacitance
value with a lower control voltage, the sensitivity of the control
field (intensity of the control field) is desirably enhanced by
making the ferroelectric layer 205 thinner. However, making the
ferroelectric layer 205 thinner causes a problem that the
insulating properties are decreased and the withstanding voltage
relative to the signal voltage becomes lower.
[0017] That is, in the varactor element 200 having such an
electrode configuration as proposed in Japanese Unexamined Patent
Application Publication No. 2007-287996, to make the control
voltage lower is incompatible to make the withstanding voltage
relative to the signal voltage higher. Accordingly, it is difficult
for the varactor element 200 having such an electrode configuration
as proposed in Japanese Unexamined Patent Application Publication
No. 2007-287996 to sufficiently support such an application in
which a signal voltage with large amplitude is inputted and the
capacitance value of the varactor element 200 is desired to be
controlled with a lower control voltage. In a case that the
intensity of the signal field is large, there is also a problem of
varying the permittivity depending on the level of a signal
producing the signal field (alternating current signal) and thus
changing the capacitance of the varactor element.
[0018] It is desirable to provide a varactor element that can
establish the sensitivity of a control field between the control
terminals regardless of the withstanding voltage performance
relative to the signal voltage and maintains capacitance unchanged
by the level of a signal inputted between the signal terminals and
to provide an electronic device provided with the same.
[0019] According to an embodiment, there is provided a first
varactor element provided with a dielectric layer; a pair of signal
electrodes, each disposed on one face of the dielectric layer and
facing each other; and a pair of control electrodes, each disposed
on another face of the dielectric layer. In the embodiment, the
pair of control electrodes is disposed facing each other parallel
to a direction intersecting a direction of the pair of signal
electrodes facing each other.
[0020] According to another embodiment, there is provided a second
varactor element provided with a pair of signal electrodes disposed
facing each other parallel to a predetermined direction and a pair
of control electrodes disposed facing each other parallel to a
direction intersecting the predetermined direction. In addition,
the second varactor element according to the embodiment is provided
with a first dielectric layer disposed between the pair of signal
electrodes and disposed between the pair of control electrodes and
a second dielectric layer having a permittivity lower than that of
the first dielectric layer and disposed between the signal
electrodes and the control electrodes.
[0021] As described above, in the first and second varactor
elements according to the embodiments, a pair of signal electrodes
and a pair of control electrodes are provided separately and a
capacitance value of a signal capacitor between the pair of signal
electrodes is changed by a control field generated between the pair
of control electrodes. In addition, in the embodiments, each
electrode is disposed in such a manner that directions of the pair
of signal electrodes facing each other intersect directions of the
pair of control electrodes facing each other. Therefore, in the
embodiments, it is possible to establish a relative permittivity,
an electrode area, and an electrode distance, which are three
elements determining the capacitance, separately for the signal
electrodes and the control electrodes. That is, in the embodiments,
it is possible to separately design the withstanding voltage
performance between the signal terminals and the withstanding
voltage performance between the control terminals.
[0022] According to still another embodiment, there is provided a
first electronic device provided with the first varactor element
according to the above embodiment and a control voltage supply unit
supplying a control voltage to the pair of control electrodes.
[0023] According to yet another embodiment, there is provided a
second electronic device provided with the second varactor element
according to the above embodiment and a control voltage supply unit
supplying a control voltage to the pair of control electrodes.
[0024] In the varactor elements according to the embodiments, as
described above, it is possible to separately (independently)
design the withstanding voltage performance between the signal
terminals and the withstanding voltage performance between the
control terminals. Therefore, according to the embodiments, it is
possible to provide a varactor element that can establish the
sensitivity of a control field between the control terminals
regardless of the withstanding voltage performance relative to the
signal voltage and maintains the capacitance unchanged by the level
of a signal inputted between the signal terminals.
[0025] In addition, since the electronic devices according to the
embodiments are provided with the varactor elements according to
the embodiments described above, it is possible to control the
capacitance of the varactor element with a lower control voltage
even in an electronic device to which a high signal voltage is
inputted.
[0026] Additional features and advantages are described herein, and
will be apparent from the following Detailed Description and the
figures.
BRIEF DESCRIPTION OF THE FIGURES
[0027] FIG. 1 is a schematic configuration diagram of a varactor
element according to an embodiment;
[0028] FIG. 2 is a perspective view of a dielectric member
according to the embodiment;
[0029] FIG. 3A is a transparent view of the dielectric member
according to the embodiment taken from an upper face thereof, and
FIG. 3B is a cross-sectional view taken from IIIB-IIIB in FIG.
3A.
[0030] FIG. 4 is an exploded perspective view of the dielectric
member according to the embodiment;
[0031] FIG. 5 illustrates a behavioral outline of the varactor
element according to the embodiment;
[0032] FIG. 6 is an exploded side view of a dielectric member of a
varactor element intersecting parallel to the thickness;
[0033] FIGS. 7A through 7C are top views of individual layers
configuring the dielectric member of the varactor element
intersecting parallel to the thickness;
[0034] FIG. 8 is a schematic cross-sectional view of the dielectric
member of the varactor element intersecting parallel to the
thickness;
[0035] FIG. 9 illustrates a state of stray capacitance generated
between electrodes of a varactor element intersecting in plane;
[0036] FIG. 10A is an external perspective view of a dielectric
member of a varactor element according to another embodiment, and
FIG. 10B is a side view thereof;
[0037] FIG. 11 is a layout drawing of signal electrodes and control
electrodes in the varactor element according to the other
embodiment;
[0038] FIG. 12 is a layout drawing of low dielectric layers in the
varactor element according to the other embodiment;
[0039] FIG. 13 is an exploded perspective view of the dielectric
member in the varactor element according to the other
embodiment;
[0040] FIG. 14A is a top view of a mask used for forming the signal
electrodes and the control electrodes of the varactor element
according to the other embodiment, and FIG. 14B is a top view of a
mask used for forming the low dielectric layers thereof;
[0041] FIGS. 15A and 15B illustrate another configuration example
of a mask used for forming the signal electrodes and the control
electrodes;
[0042] FIG. 16 illustrates a behavioral outline of the varactor
element according to the other embodiment;
[0043] FIG. 17 illustrates a configuration of signal electrodes,
control electrodes, and low dielectric layers of a varactor element
according to a first modification;
[0044] FIG. 18 illustrates a configuration of signal electrodes,
control electrodes, and low dielectric layers of a varactor element
according to a second modification;
[0045] FIG. 19A is an exploded perspective view of a dielectric
member of a varactor element according to a third modification, and
FIG. 19B illustrates a configuration of signal electrodes, control
electrodes, and low dielectric layers of the varactor element
according to the third modification;
[0046] FIG. 20 illustrates a state of stray capacitance generated
between the signal electrodes and the control electrodes in a
varactor element intersecting parallel to the thickness;
[0047] FIG. 21A is an exploded perspective view of a dielectric
member of a varactor element according to a fourth modification,
and FIG. 21B is an exploded side view of the dielectric member
according to the fourth modification;
[0048] FIGS. 22A through 22E are top views of individual layers
configuring the dielectric member according to the fourth
modification;
[0049] FIG. 23A is a schematic cross-sectional view of the
dielectric member according to the fourth modification, and FIG.
23B is a transparent view of the dielectric member of the varactor
element according to the fourth modification taken from an upper
face thereof;
[0050] FIG. 24A is an exploded perspective view of a dielectric
member of a varactor element according to a fifth modification, and
FIG. 24B is an exploded side view of the dielectric member
according to the fifth modification;
[0051] FIG. 25 is a schematic cross-sectional view of the
dielectric member according to the fifth modification;
[0052] FIG. 26 is a schematic circuit configuration of the
electronic device according to a still another embodiment;
[0053] FIGS. 27A and 27B are circuit configurations in the vicinity
of varactor capacitors in the past;
[0054] FIG. 28A is a schematic perspective view of a varactor
element in the past, and FIG. 28B is a cross-sectional
configuration diagram of the varactor element in the past; and
[0055] FIG. 29 illustrates a behavioral outline of the varactor
element in the past.
DETAILED DESCRIPTION
[0056] A description is given below to examples of varactor
elements according to embodiments and an electronic device provided
with the same referring to the drawings in the following order.
Embodiments are not limited to the examples below.
[0057] 1. Embodiment: a basic configuration example of a varactor
element according to an embodiment
[0058] 2. Another Embodiment: a configuration example of a varactor
element provided with low dielectric layers
[0059] 3. Still Another Embodiment: a configuration example of an
electronic device provided with a varactor element according to
embodiments
1. Embodiment
[0060] Configuration of Varactor Element
[0061] FIG. 1 illustrates a schematic configuration of a varactor
element according to an embodiment. In FIG. 1, the direction from
left to right of the drawing is defined as an x direction, the
direction from bottom to top of the drawing as a z direction, and
the direction from front to back of the drawing as a y
direction.
[0062] A varactor element 10 is provided with a dielectric member 1
in, for example, a rectangular parallelepiped shape, a pair of
signal terminals 2a and 2b (first and second signal terminals), and
a pair of control terminals 3a and 3b (first and second control
terminals).
[0063] Both of the first and second signal terminals 2a and 2b can
be formed of a metal member having the shape of, for example, a
flat plate. The first and second signal terminals 2a and 2b are
provided respectively on two facing side faces of the dielectric
member 1. In the example illustrated in FIG. 1, the first and
second signal terminals 2a and 2b are provided respectively on the
two side faces that are perpendicular to the x direction. The first
and second signal terminals 2a and 2b are connected respectively to
first and second signal electrodes 12a and 12b, described later,
formed in the dielectric member 1.
[0064] Both of the first and second control terminals 3a and 3b can
be formed of a metal member having the shape of, for example, a
flat plate. The first and second control terminals 3a and 3b are
provided respectively on two facing side faces of the dielectric
member 1 on which the first and second signal terminals 2a and 2b
are not provided. In the example illustrated in FIG. 1, the first
and second control terminals 3a and 3b are provided respectively on
the two side faces that are perpendicular to the y direction. The
first and second control terminals 3a and 3b are connected
respectively to first and second control electrodes 13a and 13b,
described later, formed in the dielectric member 1. In addition,
the signal terminals and the control terminals are formed on the
side faces of the dielectric member 1 so as not to make contact
with each other.
[0065] FIGS. 2, 3A, and 3B illustrate a configuration example of
the dielectric member according to this embodiment. FIG. 2 is an
external perspective view of the dielectric member 1. FIG. 3A is a
transparent view taken from the upper face of the dielectric member
1, and FIG. 3B is a cross-sectional view taken from the IIIB-IIIB
in FIG. 3A.
[0066] The dielectric member 1 is configured mainly with a
dielectric member main body 11, and a pair of signal electrodes 12a
and 12b (first and second signal electrodes) and a pair of control
electrodes 13a and 13b (first and second control electrodes) formed
therein.
[0067] The dielectric member main body 11 is formed of a dielectric
material having a permittivity varied by an application of the
control voltage. For example, the dielectric member main body 11 is
formed of a ferroelectric material with such a relative
permittivity of more than 1000.
[0068] Specifically, it is possible to use a ferroelectric material
that produces ionic polarization for the material to form the
dielectric member main body 11. The ferroelectric material
producing ionic polarization is a ferroelectric material that is
formed of an ionic crystalline material and electrically polarizes
due to displacement of positive and negative ion atoms. Such a
ferroelectric material producing ionic polarization is generally
represented by a chemical formula, ABO.sub.3 (O denotes an oxygen
element), where A and B denote predetermined two elements, and has
a perovskite structure. Such a ferroelectric material may include,
for example, barium titanate (BaTiO.sub.3), potassium niobate
(KNbO.sub.3), and lead titanate (PbTiO.sub.3). As a material to
form the dielectric member main body 11, PZT (lead zirconate
titanate), for example, may also be used in which lead titanate
(PbTiO.sub.3) is mixed with lead zirconate (PbZrO.sub.3).
[0069] As a material to form the dielectric member main body 11, a
ferroelectric material may also be used that produces electronic
polarization. In such a ferroelectric material, an electric dipole
moment is produced in which a portion biased by positive charge is
divided from a portion biased by negative charge and thus the
polarization is produced. As such a material, rare earth iron
oxides has been reported in the past that form a surface of
Fe.sup.2+ charge and surface of Fe.sup.3+ charge to form the
polarization and show ferroelectric properties. In this system,
materials represented by a molecular formula,
(RE).(TM).sub.2.O.sub.4 (O: oxygen element), where RE denotes a
rare earth element and TM denotes an iron group element are
reported to have a high permittivity. Such a rare earth element may
include, for example, Y, Er, Yb, and Lu (in particular, Y and heavy
rare earth element), and such an iron group element may include,
for example, Fe, Co, and Ni (in particular, Fe). Such
(RE).(TM).sub.2.O.sub.4 may include, for example,
ErFe.sub.2O.sub.4, LuFe.sub.2O.sub.4, and YFe.sub.2O.sub.4.
[0070] As a material to form the dielectric member main body 11, a
ferroelectric material having anisotropy may also be used. The
dielectric member main body 11 is formed by integrating a plurality
of dielectric sheets described later by sintering or the like.
[0071] The first and second signal electrodes 12a and 12b are
electrodes to which the alternating current signal is applied from
the outside via the first and second signal terminals 2a and 2b,
and are disposed to face each other separated at a predetermined
interval. In the example illustrated in FIGS. 2 and 3, both of the
first and second signal electrodes 12a and 12b are configured with
electrodes having an upper face in the shape of, for example, a
rectangular, and both signal electrodes are disposed in such a
manner that both signal electrodes have each end on a transverse
side facing each other separated at a predetermined interval. In
the example illustrated in FIGS. 2 and 3, the first and second
signal electrodes 12a and 12b are disposed in such a manner that
they face parallel to the x direction.
[0072] The first and second control electrodes 13a and 13b are
electrodes to which the controlling voltage is applied via the
first and second control terminals 3a and 3b, and are disposed to
face each other separated at a predetermined interval. At this
point, the first and second control electrodes 13a and 13b are
disposed in such a manner that the directions of the first and
second control electrodes 13a and 13b facing each other intersect
the directions of the first and second signal electrodes 12a and
12b.
[0073] In the example illustrated in FIGS. 2 and 3, both first and
second control electrodes 13a and 13b are configured with
electrodes having an upper face in the shape of, for example, a
rectangular, and both control electrodes are disposed in such a
manner that both control electrodes have each end on a transverse
side facing each other separated at a predetermined interval. In
the example illustrated in FIGS. 2 and 3, the first and second
control electrodes 13a and 13b are disposed in such a manner that
the directions of the first and second control electrodes 13a and
13b facing each other are perpendicular to the directions of the
first and second signal electrodes 12a and 12b facing each other (x
direction), that is, become parallel to the y direction.
[0074] In addition, the first and second signal electrodes 12a and
12b and the first and second control electrodes 13a and 13b are
formed in a thickness comparable to each other and are formed on an
approximately identical plane in the in-plane direction of the
dielectric member 1.
[0075] This embodiment is assumed to provide the varactor element
10 suitable for an application to desirably have a large external
signal inputted thereto and change the capacitance at a low control
voltage. Therefore, a distance ds between the facing first and
second signal electrodes 12a and 12b (electrode distance) is
preferably established to be larger than a distance dr between the
facing first and second control electrodes 13a and 13b as
illustrated in FIG. 3A. This is because of the following
reason.
[0076] Since an input signal is large in such an application as
described above, from the perspective of the resistance properties
in the varactor element 10, the distance between the signal
electrodes is preferably extended as much as possible to make the
intensity of the signal field in the element smaller. In addition,
from the perspective of low voltage driving, the distance between
the control electrodes is preferably established as small as
possible to enhance the intensity of the control field. Therefore,
in such an application as described above, the distance ds between
the signal electrodes are usually larger than the distance dr
between the control electrodes as illustrated in FIG. 3A. However,
the relative magnitudes of the distance ds between the signal
electrodes and the distance dr between the control electrodes are
not limited to that but can be appropriately modified in
correspondence with applications. That is, depending on
applications to apply the varactor element 10, there are also cases
that the distance ds between the signal electrodes is same as the
distance dr between the control electrodes or the distance ds
between the signal electrodes is smaller than the distance dr
between the control electrodes.
[0077] The first and second signal electrodes 12a and 12b and the
first and second control electrodes 13a and 13b are formed using a
conductive paste including, for example, metal fine powder (Pd,
Pd/Ag (alloy of Pd and Ag), Ni, or the like). This enables to
reduce the costs of manufacturing the varactor element 10.
[0078] By disposing and configuring each of the electrodes as
described above, it becomes possible to intersect the direction of
the signal field generated between the first and second signal
electrodes 12a and 12b with the direction of the control field
generated between the first and second control electrodes 13a and
13b in an identical plane in the dielectric member 1.
[0079] As the signal field and the control field intersect with
each other in the in-plane direction of the dielectric member 1 in
the varactor element 10 according to this embodiment as described
above, it is referred to below as a varactor element 10
intersecting in plane. In this embodiment, a dielectric region 14
of the dielectric member main body 11 sandwiched between the first
and second signal electrodes 12a and 12b and the first and second
control electrodes 13a and 13b becomes a main region having
variable capacitance. Therefore, the dielectric region 14 is
referred to below as a variable capacitance region (first
dielectric layer).
[0080] Although a description is given above to the configuration
example in which the directions of the signal electrodes facing
each other is perpendicular to the directions of the control
electrodes facing each other in this embodiment, embodiments are
not limited to this. The directions of the signal electrodes facing
each other and the directions of the control electrodes facing each
other may not be perpendicular to but may also intersect with each
other. In this embodiment, a description is given above with FIGS.
2 and 3 to the example in which the pair of signal electrodes are
provided so as to face each other parallel to the longitudinal
sides (x direction in the drawings) of the dielectric member main
body 11 and the pair of control electrodes are provided so as to
face each other parallel to the transverse sides (y direction in
the drawings) of the dielectric member main body 11. However,
embodiments are not limited to this. In FIGS. 2 and 3, the pair of
signal electrodes may also face each other in the y direction and
the pair of control electrodes in the x direction. Although a
description is given above to the example of the dielectric member
1 in, for example, a rectangular parallelepiped shape in this
embodiment, the shape of the dielectric member 1 can be arranged in
any shape in correspondence with applications and the like.
[0081] Method of Fabricating Varactor Element
[0082] Next, a brief description is given to an example of a method
of fabricating the varactor element 10 according to this embodiment
referring to FIG. 4. FIG. 4 is an exploded perspective view of the
dielectric member 1 according to this embodiment.
[0083] Firstly, a dielectric sheet 11a (dielectric layer) formed of
the ferroelectric material described above is prepared. In
addition, a mask is prepared in which openings are formed in
regions corresponding to regions to form the first and second
signal electrodes 12a and 12b and the first and second control
electrodes 13a and 13b.
[0084] Subsequently, a conductive paste is prepared by making metal
fine powder of, for example, Pd, Pd/Ag, Ni, or the like into a
paste, and the conductive paste is applied (silk screen printing or
the like) on the dielectric sheet 11a via the mask. In this way,
the first and second signal electrodes 12a and 12b and the first
and second control electrodes 13a and 13b are formed on one of the
surfaces of the dielectric sheet 11a.
[0085] Subsequently, another dielectric sheet 11b formed of the
ferroelectric material described above is laminated on the surface
of the dielectric sheet 11a on which the electrodes are formed for
thermocompression. Then, the thermocompressed members are sintered
in a reducing atmosphere at a high temperature to integrate the two
dielectric sheets 11a and 11b and the conductive paste layer
(signal electrodes and control electrodes). In this embodiment, the
varactor element 10 intersecting in plane is fabricated in such a
manner.
[0086] Behavior of Varactor Element
[0087] FIG. 5 is a schematic view illustrating a behavioral outline
of the varactor element 10 according to this embodiment. As
described above, in the varactor element 10 according to this
embodiment, the direction of the signal field generated between the
first and second signal terminals 2a and 2b is perpendicular to the
direction of the control field generated between the first and
second control terminals 3a and 3b. Therefore, the varactor element
10 according to this embodiment has a configuration equivalent to a
configuration in which each terminal is disposed in such a manner
that the directions of the pair of signal terminals 2a and 2b
facing each other is perpendicular to the directions of the pair of
control terminals 3a and 3b facing each other and a separate
varactor capacitor is connected between the signal terminals and
between the control terminals respectively.
[0088] In the varactor element 10 according to this embodiment,
since the dielectric member 1 is formed of the ferroelectric
material described above, the state of polarization in the variable
capacitance region 14 is changed by applying a direct current
control voltage between the first and second control terminals 3a
and 3b and thus the permittivity (relative permittivity) is varied.
As a result, the capacitance value Cr of the controlling capacitor
between the first and second control terminals 3a and 3b. At this
point, with the variation of the permittivity in the variable
capacitance region 14, the capacitance value Cs between the first
and second signal terminals 2a and 2b is also changed. That is, in
the varactor element 10 according to this embodiment, the
capacitance value Cs between the first and second signal terminals
2a and 2b can be controlled by the control voltage applied between
the first and second control terminals 3a and 3b. Therefore, in
this embodiment, the varactor element 10 can be acted as a
capacitive element having a controlled signal capacitor in a case
that the alternating current signal is inputted between the signal
terminals.
[0089] As described above, in the varactor element 10 according to
this embodiment, the pair of signal electrodes 12a and 12b
(terminals) and the pair of control electrodes 13a and 13b
(terminals) are provided separately. In this embodiment, each of
the electrodes is disposed in such a manner that the directions of
the pair of signal electrodes 12a and 12b facing each other and the
directions of the pair of control electrodes 13a and 13b facing
each other become perpendicular to each other. Then, the
capacitance value of the signal capacitor between the pair of
signal electrodes 12a and 12b is controlled by the control field
generated between the pair of control electrodes 13a and 13b.
Therefore, in this embodiment, the relative permittivity, the
electrode area, and electrode distance, which are the three
elements determining the capacitance, can be separately established
for the signal electrodes and the control electrodes respectively.
That is, according to this embodiment, the withstanding voltage
performance between the signal terminals 2a and 2b and the
withstanding voltage performance between the control terminals 3a
and 3b can be separately designed, and thus the sensitivity of the
control field between the control terminals 3a and 3b can be
established regardless of the withstanding voltage relative to the
signal voltage. In addition, according to this embodiment, it is
possible to provide a varactor element that maintains the
capacitance unchanged by the level of the signal inputted between
the signal terminals.
[0090] As a varactor element having a configuration of intersecting
the signal field and the control field other than this embodiment,
a configuration, for example, in which a signal field is generated
in an in-plane direction of a dielectric member and a control field
is generated parallel to the thickness of the dielectric member is
also considered. FIG. 6 illustrates a configuration of such a
varactor element. FIG. 6 is an exploded side view of a dielectric
member of such a varactor element.
[0091] A dielectric member 20 of a varactor element illustrated in
FIG. 6 is provided with a first dielectric sheet 21 having a first
control electrode 22 formed on the surface, a second dielectric
sheet 23 having a pair of signal electrodes 24 and 25 formed on the
surface, and a third dielectric sheet 26 having a second control
electrode 27 formed on the surface. The dielectric member 20 is
further provided with a fourth dielectric sheet 28 that covers the
second control electrode 27 formed on the third dielectric sheet
26. Each dielectric sheet can be formed of a ferroelectric material
similar to the material to form the dielectric member described in
the above embodiment. Each electrode can also be formed of a
material similar to the material to form the signal electrodes and
the control electrodes according to this embodiment described
above.
[0092] FIGS. 7A through 7C illustrate a configuration of the
electrodes formed on each of the dielectric sheets. FIG. 7A is a
top view of the third dielectric sheet 26, FIG. 7B is a top view of
the second dielectric sheet 23, and FIG. 7C is a top view of the
first dielectric sheet 21.
[0093] On the third dielectric sheet 26, as illustrated in FIG. 7A,
the second control electrode 27 is formed that extends along a
direction from the center at an end on one of the longitudinal
sides parallel to the transverse sides (y direction in FIG. 7A) and
has an upper face in, for example, a rectangular shape.
[0094] On the second dielectric sheet 23, as illustrated in FIG.
7B, a pair of signal electrodes 24 and 25 having an upper face in,
for example, a rectangular shape is formed in the vicinity of both
ends on the transverse sides separated at a predetermined interval.
The distance between the pair of signal electrodes 24 and 25 is
established at a value equal to or more than the width of the
second control electrode 27 (or the first control electrode 22). In
the example of FIG. 7B, the configuration (shape, dimensions, and
the like) of the pair of signal electrodes 24 and 25 is same.
[0095] On the first dielectric sheet 21, as illustrated in FIG. 7C,
the first control electrode 22 is formed that extends along a
direction from the center at an end on the other of the
longitudinal sides parallel to the transverse sides (y direction in
FIG. 7C) and has an upper face in, for example, a rectangular
shape. In the example of FIGS. 7A and 7C, the first and second
control electrodes 22 and 27 have a same configuration.
[0096] FIG. 8 illustrates a schematic cross-sectional view of the
dielectric member 20 after integrating each of the dielectric
sheets. As the electrodes of such a configuration as described
above are formed on the respective dielectric sheets and the sheets
are laminated in the order illustrated in FIG. 6 for integration,
the first and second control electrodes 22 and 27 are disposed
facing each other parallel to the thickness of the dielectric
member 20. In contrast, the pair of signal electrodes 24 and 25 is
disposed facing each other on an identical plane in the in-plane
direction of the dielectric member 20.
[0097] In the varactor element illustrated in FIGS. 6 through 8,
since the signal field is directed to the in-plane direction of the
dielectric member 20 and the control field is directed parallel to
the thickness, it is possible to intersect the signal field with
the control field. Therefore, in a varactor element with such a
configuration, the withstanding voltage performance between the
signal terminals and the withstanding voltage performance between
the control terminals can also be designed separately and the
sensitivity of the control field between the control terminals can
also be established regardless of the withstanding voltage
performance relative to the signal voltage. A varactor element
having such a mode of field intersection as the example illustrated
in FIGS. 6 through 8 is referred to below as a varactor element
intersecting parallel to the thickness.
[0098] However, the varactor element intersecting parallel to the
thickness described above has the following disadvantages. This
type of varactor element has a high accuracy of printing when
forming predetermined electrodes on each of the dielectric sheets.
Compared to that, it has a low accuracy of alignment between the
electrodes when laminating each of the dielectric sheets.
Therefore, in a varactor element intersecting parallel to the
thickness, it becomes difficult to make the electrodes larger in
consideration of the misalignment between the electrodes when
laminating each of the dielectric sheets. For example, in the
example illustrated in FIGS. 6 through 8, it is difficult to make
the width of the two control electrodes 22 and 27 sufficiently
widely. That is, in a varactor element intersecting parallel to the
thickness, the size of the electrodes is limited due to the
misalignment between the electrodes when laminating each of the
dielectric sheets and there is a possibility that it is difficult
to make the variable amount of the capacitance sufficiently
large.
[0099] In the varactor element intersecting parallel to the
thickness, the thickness d between the pair of control electrodes
22 and 27 becomes a thickness of approximately two dielectric
sheets (refer to FIG. 8). That is, in the varactor element
intersecting parallel to the thickness, it is difficult to make the
interval between the control electrodes sufficiently small because
of the restriction due to the number of dielectric sheets in
between and the thickness of each of the dielectric sheets. As a
result, in the varactor element intersecting parallel to the
thickness, there is a possibility that it becomes difficult to make
the control voltage sufficiently small.
[0100] In contrast, in the varactor element 10 according to this
embodiment, since the signal electrodes and the control electrodes
are formed in an identical plane in the dielectric member 1, such
disadvantages described above (misalignment during the lamination)
can be eliminated.
[0101] A configuration of the varactor element intersecting
parallel to the thickness may be a configuration having a region in
which a pair of control electrodes overlaps taken from the upper
face of the varactor element as in the example illustrated in FIGS.
6 through 8 and may also be a configuration in which a pair of
control electrodes does not overlap. As the no overlap
configuration, the following configuration, for example, is
considered. The mode of disposition of a pair of control electrodes
and a pair of signal electrodes taken from the upper face of the
varactor element is made similar to that of this embodiment (FIGS.
3A and 3B) and also each of the control electrodes is formed
respectively in a dielectric layer upper or lower than the
dielectric layer in which the pair of signal electrodes is formed.
In such a configuration, the direction of the control field
produced between the pair of control electrodes is tilted relative
to the thickness and in-plane directions of the varactor element.
In this case, although the distance between the pair of control
electrodes becomes larger than the distance in the example
illustrated in FIGS. 6 through 8, the distances between the control
electrodes and the signal electrodes also becomes larger compared
to the example illustrated in FIGS. 6 through 8, so that the
parasitic capacitance between the control electrodes and the signal
electrodes can be made smaller compared to the example illustrated
in FIGS. 6 to 8.
2. Another Embodiment
[0102] In the varactor element according to the above embodiment, a
pair of signal electrodes and a pair of control electrodes are
formed on an approximately identical plane in the in-plane
direction in the dielectric member, and as the distance between the
signal electrodes and the control electrodes becomes smaller, stray
capacitance is generated between the signal electrodes and the
control electrodes. Such a state is illustrated in FIG. 9. FIG. 9
is a transparent view of the dielectric member of the varactor
element according to the previous embodiment taken the upper
face.
[0103] In the varactor element according to the previous
embodiment, when the distance between the signal electrodes and the
control electrodes becomes smaller, stray capacitance (parasitic
capacitance) is generated between the signal electrodes and the
control electrodes in a region other than the variable capacitance
region 14 as illustrated with the arrows in FIG. 9. More
specifically, the stray capacitance is generated between ends on
longitudinal sides of the signal electrodes and ends on
longitudinal sides of the control electrodes closest to the ends of
the signal electrodes. When the stray capacitance is generated
between the signal electrodes and the control electrodes in such a
manner, the range of variation in the capacitance of the varactor
element becomes smaller.
[0104] With that, in this embodiment, a description is given to a
varactor element in which the withstanding voltage performance
between the signal terminals and the withstanding voltage
performance between the control terminals can be designed
separately and also the above problem of stray capacitance
generated between the signal electrodes and the control electrodes
can also be eliminated.
[0105] Configuration of Varactor Element
[0106] The entire configuration of a varactor element according to
this embodiment is similar to the configuration of the previous
embodiment (FIG. 1), though not shown, and the varactor element
according to this embodiment is provided with a dielectric member
in, for example, a rectangular parallelepiped shape, a pair of
signal terminals, and a pair of control terminals. The disposition
of each terminal is also similar to the previous embodiment.
However, in this embodiment, only the configuration of the
dielectric member is modified from that of the previous embodiment.
Therefore, in this section, a description is given only to the
configuration of the dielectric member.
[0107] FIGS. 10A and 10B illustrate a configuration example of the
dielectric member according to this embodiment. FIG. 10A an
external perspective view of the dielectric member, and FIG. 10B is
a side view of the dielectric member taken from the y direction in
FIG. 10A. In FIGS. 10A and 10B, components similar to the previous
embodiment (FIG. 2) are assigned to the same reference numerals and
characters. In FIG. 10A, the direction from left to right of the
drawing is defined as an x direction, the direction from bottom to
top of the drawing as a z direction, and the direction from front
to back of the drawing as a y direction.
[0108] A dielectric member 31 is configured with a dielectric
member main body 11, and a pair of signal electrodes 12a and 12b
(first and second signal electrodes), a pair of control electrodes
13a and 13b (first and second control electrodes), and four low
dielectric layers 32 (second dielectric layers) formed therein.
[0109] The dielectric member main body 11 is formed of a
ferroelectric material with such a relative permittivity of more
than 1000, for example, similar to the previous embodiment. The
first and second signal electrodes 12a and 12b and the first and
second control electrodes 13a and 13b have a configuration similar
to the previous embodiment and are disposed similarly.
[0110] FIG. 11 illustrates a layout drawing of the upper faces of
the first and second signal electrodes 12a and 12b and the first
and second control electrodes 13a and 13b. The first and second
signal electrodes 12a and 12b are disposed along the x direction in
the drawing separated by a predetermined distance, and the first
and second control electrodes 13a and 13b are disposed along the y
direction in the drawing separated by a predetermined distance. The
first and second signal electrodes 12a and 12b and the first and
second control electrodes 13a and 13b are disposed on an
approximately identical plane in the in-plane direction of the
dielectric member 31. In this embodiment, a distance ds between the
facing first and second signal electrodes 12a and 12b is configured
to be larger than a distance dr between the facing first and second
control electrodes 13a and 13b. However, embodiments are not
limited to this but the distance ds between the facing signal
electrodes may also be equal to or less than the distance dr
between the facing control electrodes (ds.ltoreq.dr) in
correspondence with applications and the like.
[0111] The example of disposition of the pair of signal electrodes
12a and 12b and the pair of control electrodes 13a and 13b are not
limited to this. The directions of the signal electrodes facing
each other and the directions of the control electrodes facing each
other may not be perpendicular to but may also intersect with each
other.
[0112] The four low dielectric layers 32 is a layer having an upper
face in, for example, a rectangular shape and a layer to reduce the
stray capacitance between the signal electrodes and the control
electrodes described above. Each of the low dielectric layers 32 is
formed of a dielectric material having a permittivity (relative
permittivity) sufficiently smaller than the permittivity (relative
permittivity) of the dielectric member main body 11. For example,
it is formed of a dielectric material having a relative
permittivity of, for example, 10 or less. As a material to form the
low dielectric layers 32, it is preferred to use a material that
can be formed by application (silk screen printing and the like)
similar to each of the electrodes from the perspective of easy
manufacture of the varactor element. The dielectric material
satisfying these conditions may include, for example, dielectric
materials of alumina system, magnesium silicate system, and the
like. In this embodiment, the four low dielectric layers 32 have a
thickness comparable to the thicknesses of the signal electrodes
and the control electrodes.
[0113] FIG. 12 illustrates a layout drawing of the upper face of
the four low dielectric layers 32 in this embodiment. In this
embodiment, the four low dielectric layers 32 are disposed on a
plane approximately identical to the pair of signal electrodes 12a
and 12b and the pair of control electrodes 13a and 13b in the
in-plane direction of the dielectric member 31. The four low
dielectric layers 32 are formed in a region excluding the pair of
signal electrodes 12a and 12b, the pair of control electrodes 13a
and 13b, and the variable capacitance region 14 (first dielectric
layer). More specifically, in this embodiment, the low dielectric
layers 32 are formed in a region surrounded by the ends on the
longitudinal sides of the signal electrodes, ends on the
longitudinal sides of the control electrodes closest to the ends of
the signal electrodes, and side faces of the dielectric member 31.
By disposing in such a manner, the stray capacitance can be reduced
that is generated between the ends on the longitudinal sides of the
signal electrodes and the ends on the longitudinal sides of the
control electrodes closest to the ends of the signal
electrodes.
[0114] However, the disposition of the low dielectric layers 32 is
not limited to the example of FIG. 12. The low dielectric layers 32
can be disposed in any region as long as it is a region in which
the stray capacitance between the signal electrodes and the control
electrodes is generated in a plane approximately same as the signal
electrodes and the control electrodes.
[0115] Method of Fabricating Varactor Element
[0116] Next, a brief description is given to a method of
fabricating the varactor element according to this embodiment
referring to FIGS. 13 and 14. FIG. 13 is an exploded perspective
view of the dielectric member 31 according to this embodiment. FIG.
14A is a top view of the mask used for forming the signal
electrodes and the control electrodes, and FIG. 14B is a top view
of the mask for forming the low dielectric layers 32.
[0117] Firstly, a first dielectric sheet 11a (dielectric layer) is
prepared that is formed of a ferroelectric material similar to the
material to form a dielectric member described in the previous
embodiment. In addition, an electrode-forming mask 33 is prepared
that is provided with openings 34 in regions corresponding to the
regions to form the signal electrodes and the control electrodes as
illustrated in FIG. 14A and a screening area 35 in a region other
than those. Further, a low dielectric layer-forming mask 36 is
prepared that is provided with openings 37 in regions corresponding
to the regions to form the low dielectric layers 32 as illustrated
in FIG. 14B and a screening area 38 in a region other than
those.
[0118] Subsequently, a conductive paste is prepared by making metal
fine powder of, for example, Pd, Pd/Ag, Ni, or the like into a
paste, and the conductive paste is applied (silk screen printing or
the like) on a first dielectric sheet 11a via the electrode-forming
mask 33. In this way, the first and second signal electrodes 12a
and 12b and the first and second control electrodes 13a and 13b are
formed on one of the surfaces of the first dielectric sheet
11a.
[0119] Subsequently, a paste material including a low dielectric
having a relative permittivity of, for example, 10 or less is
applied (silk screen printing) on the first dielectric sheet 11a
via the low dielectric layer-forming mask 36. In this way, the four
low dielectric layers 32 are formed on one of the surfaces of the
first dielectric sheet 11a on which the first and second signal
electrodes 12a and 12b and the first and second control electrodes
13a and 13b are formed.
[0120] Subsequently, a second dielectric sheet 11b that is formed
of a ferroelectric material similar to the material to form a
dielectric member described in the previous embodiment is laminated
on the surface of the first dielectric sheet 11a on which the
electrodes are formed for thermocompression. Then, the
thermocompressed member is sintered in a reducing atmosphere at a
high temperature to integrate the first and second dielectric
sheets 11a and 11b, the signal electrodes, the control electrodes,
and the low dielectric layers 32. In this embodiment, the varactor
element intersecting in plane is fabricated in such a manner.
[0121] The electrode-forming mask 33 (FIG. 14A) described above
used for fabricating the dielectric member 31 according to this
embodiment can also be configured using two masks illustrated in
FIGS. 15A and 15B. A mask 39 illustrated in FIG. 15A is a mask
provided with screening areas 40 in regions corresponding to the
regions to form the low dielectric layers 32. A mask 41 illustrated
in FIG. 15B is a mask formed with a screening area 42 in the region
corresponding to the vicinity of the variable capacitance region
14. The two masks illustrated in FIGS. 15A and 15B are used by
overlapping, and thereby a mask pattern similar to the
electrode-forming mask 33 illustrated in FIG. 14A can be
configured.
[0122] Since the mask 39 in FIG. 15A is a mask having a reversal
pattern of the openings of the low dielectric layer-forming mask 36
illustrated in FIG. 14B, the mask data for fabricating the low
dielectric layer-forming mask 36 can be utilized to fabricate the
mask 39. Therefore, in a case of configuring the electrode-forming
mask 33 using two masks illustrated in FIGS. 15A and 15B, the
electrode-forming mask is easily fabricated.
[0123] Behavior of Varactor Element
[0124] FIG. 16 illustrates a transparent view of the dielectric
member 31 in the varactor element according to this embodiment
taken from the upper face thereof. In this embodiment, similar to
the previous embodiment, the pair of signal electrodes 12a and 12b
(terminals) and the pair of control electrodes 13a and 13b
(terminals) are provided separately. In this embodiment, each of
the electrodes is disposed in such a manner that the directions of
the pair of signal electrodes 12a and 12b facing each other and the
directions of the pair of control electrodes 13a and 13b facing
each other become perpendicular to each other. Then, the
capacitance value of the signal capacitor between the pair of
signal electrodes 12a and 12b is controlled by the control field
generated between the pair of control electrodes 13a and 13b.
[0125] Therefore, in this embodiment, similar to the previous
embodiment, the relative permittivity, the electrode area, and
electrode distance, which are the three elements determining the
capacitance, can be separately established for the signal
electrodes and the control electrodes respectively. That is,
according to this embodiment, the withstanding voltage performance
between the signal terminals and the withstanding voltage
performance between the control terminals can be separately
designed, and thus the sensitivity of the control field between the
control terminals can be established regardless of the withstanding
voltage performance relative to the signal voltage. In addition,
according to this embodiment, it is possible to provide a varactor
element that maintains the capacitance unchanged by the level of
the signal inputted between the signal terminals.
[0126] In addition, in this embodiment, since the pair of signal
electrodes 12a and 12b and the pair of control electrodes 13a and
13b are formed on an identical plane in the dielectric member 31,
the above problem generated due to the misalignment of each of the
electrodes when laminating the dielectric sheets can be eliminated
similar to the previous embodiment.
[0127] Further in this embodiment, since the low dielectric layers
32 are formed between the signal electrodes and the control
electrodes on a plane identical to the plane on which the signal
electrodes and the control electrodes are formed in the dielectric
member 31, the stray capacitance (arrows in FIG. 16) generated
between the signal electrodes and the control electrodes can be
made smaller. Therefore, in this embodiment, deterioration in the
properties of the varactor element due to the stray capacitance
(for example, a decrease of the variation range of the capacitance
and the like) can be suppressed.
[0128] First Modification
[0129] The configuration of a varactor element that reduces the
stray capacitance generated between the signal electrodes and the
control electrodes is not limited to the above configuration
example according to the embodiment described secondly. FIG. 17
illustrates another configuration example (first modification) of a
varactor element that reduces the stray capacitance generated
between the signal electrodes and the control electrodes. FIG. 17
is a transparent view of a dielectric member of a varactor element
according to the first modification taken from the upper face
thereof. In FIG. 17, components similar to the embodiment described
secondly (FIG. 16) are assigned to the same reference numerals and
characters.
[0130] As is apparent from a comparison between FIGS. 16 and 17, a
dielectric member 51 in this example has a configuration similar to
that of the embodiment described secondly other than modifying a
configuration of low dielectric layers 52. Therefore, a description
is given in this section only to the configuration of the low
dielectric layers 52.
[0131] In this example, similar to the embodiment described
secondly, the four low dielectric layers 52 are formed in regions
other than the variable capacitance region 14. The low dielectric
layers 52 are formed of a low dielectric material similar to that
of the low dielectric layers 32 according to the embodiment
described secondly.
[0132] Further in this example, each of the low dielectric layers
52 has an upper face in, for example, an approximately rectangular
shape and has a convex 52a (referred to below as a guard area 52a)
projecting towards the variable capacitance region 14 provided at a
corner area closest to the variable capacitance region 14 out of
the four corner areas. Then, the guard areas 52a are disposed
between the corner areas in which the signal electrodes face the
control electrodes.
[0133] As described above, in the varactor element according to
this example, the guard areas 52a formed of a low dielectric
material are formed in the regions of shortest distances between
the signal electrodes and the control electrodes. Therefore, in
this example, the stray capacitance generated between the signal
electrodes and the control electrodes can be reduced even more and
the deterioration in the properties of the varactor element due to
the stray capacitance (for example, a decrease of the variation
range of the capacitance and the like) can be suppressed.
[0134] Second Modification
[0135] FIG. 18 illustrates still another configuration example
(second modification) of a varactor element that reduces the stray
capacitance generated between the signal electrodes and the control
electrodes. FIG. 18 is a transparent view of a dielectric member in
a varactor element according to the second modification taken from
the upper face thereof. In FIG. 18, components similar to the first
modification (FIG. 17) are assigned to the same reference numerals
and characters.
[0136] As apparent from a comparison between FIGS. 17 and 18, a
dielectric member 61 according to this example has a configuration
similar to that of the first modification other than modifying a
configuration of a pair of signal electrodes 62a and 62b (first and
second signal electrodes) and a pair of control electrodes 63a and
63b (first and second control electrodes). Therefore, a description
is given in this section only to the configuration of the signal
electrodes and the control electrodes.
[0137] In this example, both of the first and second signal
electrodes 62a and 62b are configured with electrodes having an
upper face in, for example, approximately rectangular shape and are
disposed so as to each have an end on one of the transverse side
facing each other separated at a predetermined interval similar to
the embodiment described secondly. In the example illustrated in
FIG. 18, the first and second signal electrodes 62a and 62b face
each other parallel to the x direction.
[0138] Both of the first and second control electrodes 63a and 63b
are configured with electrodes having an upper face in, for
example, an approximately rectangular shape. Then, the first and
second control electrodes 63a and 63b are disposed so as to each
have an end on one of the transverse side facing each other
separated at a predetermined interval and also are disposed
perpendicular to the directions of the first and second signal
electrodes 62a and 62b facing each other similar to the embodiment
described secondly. That is, in the example illustrated in FIG. 18,
the first and the second control electrodes 63a and 63b face each
other parallel to the y direction. Then, also in this example, the
first and second signal electrodes 62a and 62b and the first and
second control electrodes 63a and 63b are disposed in an
approximately identical plane in the in-plane direction of the
dielectric member 61 similar to the embodiment described
secondly.
[0139] Further in this example, a tip end of each electrode
positioned close to the variable capacitance region 14 is arranged
in a shape of the electrode width narrowing towards the variable
capacitance region 14 (in a convex). That is, the electrodes are in
a shape with no corner area provided in the regions in which the
signal electrodes and the control electrodes face each other
sandwiching the guard areas 52a of the low dielectric layers
52.
[0140] As described above, in the varactor element according to
this example, since the guard areas 52a of the low dielectric
layers 52 are formed in the regions of the shortest distances
between the signal electrodes and the control electrodes similar to
the first modification, the stray capacitance generated between the
signal electrodes and the control electrodes can be reduced even
more.
[0141] In this example, since no corner area is formed in the
regions of the signal electrodes and the control electrodes facing
each other sandwiching the guard areas 52a of the low dielectric
layers 52, the field concentration can be avoided between the
signal electrodes and the control electrodes in this region and the
stray capacitance can be reduced even more.
[0142] Further in this example, since the regions to form the guard
areas 52a in the low dielectric layers 52 are wider compared to
those of the first modification, the low dielectric layers 52 are
formed more easily.
[0143] Third Modification
[0144] Although descriptions are given above to the examples of
forming the low dielectric layers in the regions other than the
pair of signal electrodes, the pair of control electrodes, and the
variable capacitance region in the embodiment described secondly
and the first and second modifications, embodiments are not limited
to them. For example, low dielectric layers may also be configured
to cover a region other than the variable capacitance region. FIGS.
19A and 19B illustrate a configuration example (third modification)
of such a varactor element. FIG. 19A is an exploded perspective
view of a dielectric member of a varactor element according to this
example, and FIG. 19B is a transparent view of the dielectric
member according to this example taken from the upper face thereof.
In FIGS. 19A and 19B, components similar to the embodiment
described secondly (FIG. 13) are assigned to the same reference
numerals and characters.
[0145] As apparent from a comparison between FIGS. 19A and 13, a
dielectric member 70 in this example has low dielectric layers with
a configuration modified from that of the dielectric member 31
according to the embodiment described secondly. The configuration
other than that is similar to the embodiment described secondly.
Therefore, a description is given in this section only to a
configuration of the low dielectric layers.
[0146] The dielectric member 70 in this example is provided with
two low dielectric layers 71 and 72. In this embodiment, the two
low dielectric layers 71 and 72 have the same configuration. The
low dielectric layer 72 is a sheet form member having a width and a
length comparable to those of the second dielectric sheet 11b (or
the first dielectric sheet 11a). Then, an opening 72a is formed in
a region corresponding to the variable capacitance region 14 of the
varactor element. The thickness of the low dielectric layer 72 is
comparable to the thickness of the electrodes.
[0147] In this example, the first and second signal electrodes 12a
and 12b and the first and second control electrodes 13a and 13b are
disposed between the two low dielectric layers 71 and 72 as
illustrated in FIG. 19A. Therefore, in the dielectric member 70
according to this example, the low dielectric layers are formed so
as to wrap around the upper and lower faces and longitudinal side
faces of each of the electrodes except the variable capacitance
region 14 by integrating each layer.
[0148] As described above, in the varactor element according to
this example, the low dielectric layers are formed not only between
the side faces of the signal electrodes and the control electrodes
but also between the upper and lower faces of each electrode and
the dielectric member main body. Therefore, in the varactor element
according to this example, the stray capacitance generated between
the signal electrodes and the control electrodes via the dielectric
member main body can also be reduced. Accordingly, in this example,
the stray capacitance can be reduced even more compared to the
varactor elements in the embodiment described secondly and the
first and second modifications.
[0149] Although a description is given in the third modification to
the example of forming the low dielectric layers on the upper and
lower faces of the signal electrodes and the control electrodes,
embodiments are not limited to this. Such a low dielectric layer
may also be formed only on one of the upper and the lower faces of
the signal electrode and the control electrodes.
[0150] Fourth Modification
[0151] Although descriptions are given above to the types of
disposing the signal electrodes and the control electrodes on an
approximately identical plane in the in-plane direction of the
dielectric member, that is, the examples of forming the low
dielectric layers in the varactor elements intersecting in plane in
the embodiment described secondly and the first through third
modifications, embodiments are not limited to them. For example,
such low dielectric layers may also be applied to a varactor
element intersecting parallel to the thickness as illustrated in
FIGS. 6 through 8. In the fourth modification, a description is
given to a configuration example of a varactor element intersecting
parallel to the thickness having low dielectric layers.
[0152] Before describing specific configuration of the varactor
element according to this example, a brief description is given to
the mode of stray capacitance generation in a varactor element
intersecting parallel to the thickness. FIG. 20 illustrates a mode
of stray capacitance generation in a varactor element intersecting
parallel to the thickness. In a varactor element of this type, as
the distance between the first and second control electrodes 22 and
27 becomes shorter, the stray capacitance is generated parallel to
the thickness of the dielectric member 20 between the signal
electrodes and the control electrodes in a region other than the a
variable capacitance region 30 as illustrated by the arrows of
solid lines in FIG. 20. In this case, similar to the varactor
elements intersecting in plane, there is also a problem of, for
example, the variation range of the capacitance of the varactor
element made smaller.
[0153] FIGS. 21A and 21B illustrate a configuration example of a
varactor element intersecting parallel to the thickness according
to the fourth modification. FIG. 21A is an exploded perspective
view of a dielectric member of a varactor element according to this
example, and FIG. 21B is an exploded side view of the dielectric
member taken from the y direction in FIG. 21A.
[0154] A dielectric member 80 of the varactor element according to
this example is provided with a first dielectric sheet 81 having a
first control electrode 82 formed on the surface, a second
dielectric sheet 83 having a pair of low dielectric layers 84
formed on the surface, and a third dielectric sheet 85 having a
pair of signal electrodes 86 and 87 formed on the surface. The
dielectric member 80 is further provided with a fourth dielectric
sheet 88 having a pair of low dielectric layers 89 formed on the
surface, a fifth dielectric sheet 90 having a second control
electrode 91 formed on the surface, and a sixth dielectric sheet 92
covering the second control electrode 91.
[0155] In this example, the second through sixth dielectric sheets
are laminated on the first dielectric sheet 81 in this order, and
they are laminated so as to sandwich the electrodes or the low
dielectric layers between the respective dielectric sheets. Then,
in the state of laminating each of the dielectric sheets, it is
thermocompressed and sintered at a high temperature similar to the
embodiment described secondly to integrate the dielectric sheets,
the electrodes, and the low dielectric layers and form the
dielectric member 80. Each of the dielectric sheets can be formed
of a ferroelectric material similar to the material to form the
dielectric member in the embodiment described firstly. Each of the
electrodes can also be formed of a material similar to the material
to form the signal electrodes and the control electrodes of the
embodiment described firstly. Each of the low dielectric layers can
be formed of a dielectric material similar to the material to form
the low dielectric layers of the embodiment described secondly and
can also have a thickness comparable to each of the electrodes.
[0156] FIGS. 22A through 22E illustrate a configuration of the
electrodes or the low dielectric layers formed on each of the
dielectric sheets according to this example. FIG. 22A is a top view
of the fifth dielectric sheet 90, FIG. 22B is a top view of the
fourth dielectric sheet 88, and FIG. 22C is a top view of the third
dielectric sheet 85. FIG. 22D is a top view of the second
dielectric sheet 83, and FIG. 22E is a top view of the first
dielectric sheet 81.
[0157] On the fifth dielectric sheet 90, as illustrated in FIG.
22A, the second control electrode 91 is formed that extends along a
direction from the center at an end on one of the longitudinal
sides parallel to the transverse sides (y direction in FIG.
20A).
[0158] On the fourth dielectric sheet 88, as illustrated in FIG.
22B, the pair of low dielectric layers 89 is formed that has an
upper face in, for example, a rectangular shape or the like and is
separated at a predetermined interval and is disposed in the
vicinity of both ends on the transverse sides of the fourth
dielectric sheet 88. Preferably, the distance between the pair of
low dielectric layers 89 is established as equal to or more than
the width of the second control electrode 91 (or the first control
electrode 82) and also is smaller than the distance between the
pair of signal electrodes 86 and 87. In this example, the fourth
dielectric sheet 88 having the pair of low dielectric layers 89
formed on the surface has a configuration same as that of the
second dielectric sheet 83 having the pair of low dielectric layers
84 formed on the surface (refer to FIG. 22D).
[0159] On the third dielectric sheet 85, as illustrated in FIG.
22C, the pair of signal electrodes 86 and 87 is formed that has an
upper face in, for example, a rectangular shape or the like and is
separated at a predetermined interval and is disposed in the
vicinity of both ends on the transverse sides of the third
dielectric sheet 85. The distance between the pair of signal
electrodes 86 and 87 is preferably established as equal to or more
than the width of the first control electrode 82 (or the second
control electrode 91). In the example in FIG. 22C, the pair of
signal electrodes 86 and 87 has a same configuration.
[0160] On the first dielectric sheet 81, as illustrated in FIG.
22E, the first control electrode 82 is formed that extends along a
direction from the center at an end on the other of the
longitudinal sides parallel to the transverse sides (y direction in
FIG. 22E) and has an upper face in, for example, a rectangular
shape. In this example, the first and second control electrodes 82
and 91 have a same configuration.
[0161] FIGS. 23A and 23B illustrate a configuration of the
dielectric member 80 after laminating each of the dielectric sheets
for integration in this example. FIG. 23A is a schematic
cross-sectional view of the dielectric member 80, and FIG. 23B is a
transparent view of the dielectric member 80 taken from the upper
face thereof. In this example, when each of the dielectric sheets
are laminated in which the electrodes or the low dielectric layers
are formed of the configuration as illustrated in FIGS. 22A through
22E for integration, the first and second control electrodes 82 and
91 are disposed facing each other parallel to the thickness of the
dielectric member 80 (z direction in FIG. 23A). In contrast, the
pair of signal electrodes 86 and 87 are disposed facing each other
on an identical plane in the in-plane direction of the dielectric
member 80.
[0162] In this example, the low dielectric layers 84 and 89 are
formed respectively at an intermediate position between the signal
electrodes and the control electrodes parallel to the thickness of
the dielectric member 80, and the vicinities of tip ends of the low
dielectric layers 84 and 89 are disposed respectively between the
longitudinal ends of the control electrodes and the tip end of the
signal electrodes closest to the ends. In this case, the stray
capacitance can be reduced that is generated between the
longitudinal ends of the control electrodes and the tip ends of the
signal electrodes closest to the ends. Therefore, also in the
varactor element according to this example, the deterioration (for
example, a decrease in the variation range of the capacitance and
the like) in the properties of the varactor element due to the
stray capacitance generated between the signal electrodes and the
control electrodes can be suppressed even more.
[0163] In the varactor element according to this example, similar
to the above embodiments, the pair of signal electrodes 86 and 87
and the pair of control electrodes 82 and 91 are provided
separately. Each of the electrodes is disposed in such a manner
that the directions of the pair of signal electrodes 86 and 87
facing each other is perpendicular to the directions of the pair of
control electrodes 82 and 91 facing each other. Then, the control
field is generated in a direction intersecting the direction of the
signal field generated between the pair of signal electrodes 86 and
87 to control the capacitance value between the pair of signal
electrodes 86 and 87.
[0164] Therefore, also in this example, similar to the embodiments
above, the withstanding voltage performance between the signal
terminals and the withstanding voltage performance between the
control terminals can be designed separately, and thus the
sensitivity of the control field between the control terminals can
be established regardless of the withstanding voltage performance
relative to the signal voltage. In this example, similar to the
above embodiments, it is possible to provide a varactor element
that maintains the capacitance unchanged by the level of a signal
inputted between the signal terminals.
[0165] Fifth Modification
[0166] In the fifth modification, a description is given to another
configuration example of a varactor element intersecting parallel
to the thickness having low dielectric layers.
[0167] FIGS. 24A and 24B illustrate a configuration example of a
varactor element intersecting parallel to the thickness according
to the fifth modification. FIG. 24A is an exploded perspective view
of a dielectric member of a varactor element according to this
example, and FIG. 24B is an exploded side view taken from the y
direction in FIG. 24A. In FIGS. 24A and 24B, components similar to
the fourth modification (FIGS. 21A and 21B) are assigned to the
same reference numerals.
[0168] A dielectric member 95 of the varactor element according to
this example is provided with a first dielectric sheet 96 having a
first control electrode 82 formed on the surface and a second
dielectric sheet 97 having a pair of low dielectric layers 84, a
pair of signal electrodes 86 and 87, and a pair of low dielectric
layers 89 formed on the surface in this order. The dielectric
member 95 is further provided with a third dielectric sheet 98
having a second control electrode 91 formed on the surface and a
fourth dielectric sheet 99 covering the second control electrode
91.
[0169] As apparent from a comparison between FIGS. 24A and 24B and
FIGS. 21A and 21B, this example has a configuration in the
dielectric member 80 according to the fourth modification of
omitting the third and fourth dielectric sheets 85 and 88 and
directly forming the pair of signal electrodes 86 and 87 and the
pair of low dielectric layers 89 in this order on the pair of low
dielectric layers 84. The configuration other than them is similar
to that of the fourth modification.
[0170] FIG. 25 illustrates a schematic cross-sectional view of the
dielectric member 95 in this example after laminating each of the
dielectric sheets for integration. By laminating each of the
dielectric sheets described above for integration, the pair of
signal electrodes 86 and 87 are in a state of directly sandwiched
by the pair of low dielectric layers 84 and the pair of low
dielectric layers 89.
[0171] In the configuration according to this example as well,
similar to the fourth modification, the low dielectric layers 84
and 89 are formed respectively between the signal electrodes and
the control electrodes parallel to the thickness of the dielectric
member 95. Therefore, in this example as well, similar to the
fourth modification, the stray capacitance generated between the
control electrodes and the signal electrodes can be reduced by the
low dielectric layers and the influence over the properties of the
varactor element due to the stray capacitance can be
suppressed.
[0172] In addition, since the number of the dielectric sheets
between the control electrodes in the varactor element according to
this example can be less compared to the fourth modification, the
distance between the control electrodes can be shorter compared to
that in the fourth modification. Therefore, in the varactor element
according to this example, it becomes possible to drive at a lower
voltage compared to the fourth modification. However, from the
perspective of ease of method of manufacturing the varactor
element, it is easier to form each of the electrodes and the low
dielectric layers respectively on the separate dielectric sheets as
in the varactor element according to the fourth modification. That
is, the varactor element according to the fourth modification can
be manufactured more simply compared to that of the fifth
modification.
[0173] Sixth Modification
[0174] Although the configuration examples having a region in which
the two control electrodes overlap viewed from the upper face of
the varactor element are illustrated with the varactor elements
intersecting parallel to the thickness described in the fourth and
fifth modifications, embodiments are not limited to them. In the
configuration of the varactor element according to the fourth or
fifth modification, the pair of control electrodes may also be
configured in such a manner that the two control electrodes do not
overlap viewed from the upper face of the varactor element (sixth
modification).
[0175] For example, the disposition of a pair of control electrodes
and a pair of signal electrodes viewed from the upper face of a
varactor element is arranged to be similar to the embodiment
described firstly (FIGS. 3A and 3B), and the pair of the control
electrodes is formed respectively on dielectric layers upper and
lower from the dielectric layer in which the pair of signal
electrodes is formed. In this case, the direction of the control
field produced between the pair of control electrodes becomes
tilted relative to the thickness and in-plane directions of the
varactor element. Then, in such a configuration, not only effects
similar to those of the fourth and fifth modifications can be
obtained but also the distance between the control electrodes and
the signal electrodes can be larger than that of the fourth and
fifth modifications, so that the parasitic capacitance can be even
smaller.
3. Still Another Embodiment
[0176] The varactor elements according to the embodiments described
above are applicable to various electronic devices. In still
another embodiment, a description is given to a configuration
example in which a varactor element according to any of the
embodiments described above of the present application is applied
to an inverter circuit of a cold cathode fluorescent lamp (CCFL)
backlight (electronic device) used for, for example, a liquid
crystal television and the like.
[0177] FIG. 26 illustrates a schematic circuit configuration of an
inverter circuit of a CCFL backlight according to this embodiment.
An inverter circuit 100 is configured with, for example, a CCFL
101, a varactor element 102, a control voltage power source 103
(control voltage supply unit), a boost transformer 104, and a drive
circuit 105.
[0178] The varactor element 102 can employ any of the four-terminal
varactor elements described in the above embodiments and the first
through fifth modifications. Although the varactor element 102 is
illustrated as a two-terminal varactor capacitor in FIG. 26, it is
actually provided with a pair of signal terminals (not shown) and a
pair of control terminals (not shown).
[0179] One of the control terminals of the varactor element 102 is
connected to a positive electrode terminal of the control voltage
power source 103, and the other control terminal is connected to a
negative electrode terminal of the control voltage power source
103. Then, the capacitance of the varactor element 102 is
controlled by the control voltage power source 103. One of the
signal terminals of the varactor element 102 is connected to one of
the terminals of the CCFL 101, and the other signal terminal is
connected to the boost transformer 104. The varactor element 102
illustrated in FIG. 26 acts as a ballast capacitor.
[0180] The other terminal of the CCFL 101 is connected to the boost
transformer 104. The boost transformer 104 is connected by the
drive circuit 105 and is driven by the drive circuit 105.
[0181] In the inverter circuit with a configuration as described
above, a high pressure alternating current voltage that is boosted
by the boost transformer 104 is applied to the CCFL 101 via the
varactor element 102 configuring the ballast capacitor. The output
voltage of the boost transformer 104 is usually an alternating
current voltage at approximately 1500 V and 50 kHz and the current
flowing to the CCFL 101 is 5 to 10 mA.
[0182] Although a configuration provided with one CCFL 101 is
illustrated in the example of the inverter circuit illustrated in
FIG. 26, the configuration may also have two CCFLs 101 driven in
parallel. In this case, when driving the two CCFLs 101 in parallel,
the varactor element 102 is used for separation of the two CCFLs
101. As an element having such a configuration, a transformer, for
example, may also be used other than a capacitor.
[0183] Although the ballast capacitor is used in the CCFL backlight
for lower costs, it has disadvantages of variation of the
capacitance of the CCFL 101 and the differences of, for example,
the stray capacitance with a peripheral metal, resulting in the
current variation and the brightness unevenness in each CCFL
101.
[0184] In contrast to that, in this embodiment, a direct current
voltage is applied from the control voltage power source 103
connected to the control terminals of the varactor element 102
acted as a ballast capacitor to adjust the capacitance value of the
varactor element 102. Therefore, in the CCFL backlight having such
a varactor element 102 mounted therein, the brightness of the CCFL
101 can be maintained evenly by adjusting the capacitance value of
the varactor element 102.
[0185] In this embodiment, since any of the varactor elements
described in the above embodiments and the first through fifth
modifications is applied as the ballast capacitor, the varactor
element can be driven at a low control voltage while maintaining
the withstanding voltage of the signal terminals of the varactor
element. For example, the capacitance value of the varactor element
102 can be adjusted with the control voltage of approximately 1/100
to 1/300 of the output voltage of the boost transformer 104.
[0186] In the varactor element 102 according to this embodiment, as
described above, the signal terminals and the control terminals are
provided independently. Therefore, even when a direct current
voltage is applied to the control terminals of the varactor element
102 when adjusting the capacitance, the direct current voltage is
not applied to the boost transformer 104 and the CCFL 101 that are
connected to the signal terminals of the varactor element 102.
Accordingly, in this embodiment, an excessive current does not flow
in a transformer coil of the boost transformer 104 during the
capacitance adjustment, and thus the capacitance can be adjusted by
applying the control voltage to the varactor element 102 while the
varactor element 102 is implemented.
[0187] The varactor elements according to the embodiments are also
applicable to various electronic devices other than the electronic
device mentioned in the still other embodiment above, and
similarly, the capacitance can be adjusted to a desired value while
the varactor element is implemented without affecting other
circuits in the electronic device. For example, by using the
varactor element according to the embodiments, it becomes possible
to tune in tuning frequency drifts of the electronic device due to
variations in components or the like upon shipping the electronic
device.
[0188] In addition, in a case of applying any of the varactor
elements according to the embodiments to, for example, a noncontact
IC (integrated circuit) card and the like, the varactor element can
also be used as a protective circuit. More specifically, a varactor
element according to the embodiments can be used as a protective
circuit not to break the control circuit formed of semiconductor
devices having a low withstanding voltage due to an excessively
large reception signal when a noncontact IC card is moved closer to
the reader/writer. However, in this case, the direct current
voltage obtained by rectifying the received alternating current
signal is subjected to resistance division with, for example, a
circuit connecting a plurality of resistors serially in the
noncontact IC card, and the direct current voltage that is
resistance divided is used for the control voltage of the varactor
element.
[0189] That is, in the electronic device according to the
embodiments, the supply mechanism of the control voltage of the
varactor element (control voltage supply unit) is not limited to
the control voltage power source described above in the embodiment
described secondly. In case of applying the varactor element
according to the embodiments as an electronic device, such as a
noncontact IC card, for example, it is also possible to use a
circuit that generates and supplies the control voltage from the
inputted alternating current signal as a supply mechanism of the
control voltage.
[0190] It should be understood that various changes and
modifications to the presently preferred embodiments described
herein will be apparent to those skilled in the art. Such changes
and modifications can be made without departing from the spirit and
scope and without diminishing its intended advantages. It is
therefore intended that such changes and modifications be covered
by the appended claims.
* * * * *